Inasmuch as the present invention was devised to overcome specific bending problems which occur in gun tubes, the following discussion will be directed to the solution of such problems; however, it is to be understood that the concepts of the present invention are as applicable to any elongated structure which is subject to non-symmetrical thermal environments, which create non-symmetrical strains in the elongated structure.
Circumferential temperature gradients are readily established in gun tubes when exposed to non-symmetrical thermal environments, which can be produced by such factors as sunlight, wind and rain, singly or in combination. Such environments produce non-symmetrical strains in the tube, causing it to bend about its axis and, therefore, to significantly reduce the gun's firing accuracy.
This problem, of gun tube bending can be better appreciated if described analytically. The angular deflection .phi. of a beam segment of diameter D and length L due to a diametrical temperature difference .DELTA.T is given simply by: ##EQU1## where .alpha. is the linear coefficent of thermal expansion of the beam material. As an example, a typical tank gun tube has the following parameters: L=16 feet, D=6 inches, and .alpha.=6.times.10.sup.-6 per .degree.F. Using these figures, .phi.=0.197.DELTA.T mrad. This implies that a diametrical temperature gradient of only 0.5.degree. F. can induce an angular deflection of 0.1 mrad. Such temperature gradients, and indeed significantly higher ones, can readily be induced in a gun tube exposed to an asymmetrical thermal environment, such as is encountered in the field due to sun and wind.
This problem is known, and existing thermal jacket designs have been devised to attenuate these temperature gradients and, therefore, to minimize the effect of such thermal disortion by use of highly insulating materials. For example, one jacket includes alternate layers of aluminum and fiber glass wrapped around the gun tube. The purpose of the fiber glass is to provide insulation from the environment while the aluminum is used in an attempt further to reduce circumferential gradients by increasing the circumferential conductance. Such thermal jackets do reduce temperature gradients but can cause excessive heating of the tube under conditions of of rapid firing because their design is intended to provide a high radial thermal impedance between the gun tube and the environment.
A parametric study involving thermal insulating blankets having a wide range of thermal characteristics was made in the following manner.
First, a thermal jacket was assumed to have a thermal conductivity such that its total radial thermal impedance (R.sub.B) was an integer multiple of the gun tube radial thermal impedance (R.sub.G). In addition, the jacket conductance was assumed to be isotropic, i.e., the radial and circumferential thermal conductivities of the jacket were equal.
Then, for each jacket radial conductivity value, the circumferential conductivity was increased to simulate an anisotropic jacket such as might be obtained with alternate rings of an insulating material and a metal.
The parametric study covered a range of values for R.sub.B /R.sub.G from 2 to 1000. Circumferential conductivity of the jacket was limited to 156 BTU/hr-ft-.degree.F. The case for a bare gun tube was also included.
The results of the analysis of conventional thermal jackets produced some significant results and are summarized in the following conclusions. First, until the ratio (R.sub.B /R.sub.G) of the radial thermal impedance of the jacket to the radial thermal impedance of the gun tube exceeds a critical value, which depends upon gun tube dimensions, the addition of a thermal jacket will aggravate the problem of gun tube bending by thermally coupling the gun tube more, rather than less, to the external environment. This results because the increased surface area is not offset by the added thermal insulation. Accordingly, the thermal coupling to the asymmetrical thermal environment is enhanced and not reduced. Second, to be effective, an isotropic jacket of low thermal conductivity must have a very high thermal impedance where R.sub.B /R.sub.G is on the order of at least 100 to 200. As a result, excessive heating of the barrel, which is not a desirable feature, can occur when the gun is fired at a high rate, at least because gun tube wear substantially increases as the overall tube temperature increases. Moreover, it was found that an increase in the circumferential conductivity of a highly insulative jacket has a negligible effect upon the temperature gradient between opposite sides of the tube. Third, if the circumferential conductivity is high, the jacket can be very effective even if, contrary to the accepted prior art belief, the radial conductivity is high. This infers that a solid metal jacket, although not practical, would be effective in reducing gradients while at the same time allowing for good thermal dissipation under conditions of rapid fire. Therefore, a thermal jacket which would exhibit improved characteristics over those in existence should have both high radial and circumferential conductance rather than a low radial and high circumferential conductance.